1. Field of the Invention
The present invention relates generally to threaded couplings and more specifically to leak resistant threaded couplings for oilwell casing, tubing and line pipe.
2. Description of the Related Art
Presently the standard method of joining lengths of oil well casing, tubing and line pipe is by means of a threaded and coupled connection. The coupling is generally a short tubular hollow piece usually of the same material as the pipe and about 1/2 inch to 1 inch larger in outer diameter than the pipe. The typical oil well coupling is threaded on its inside diameter. Generally the coupling is threaded so that the connection may be uncoupled by unscrewing the pipe from one or both ends of the coupling. The coupling is internally threaded and is the female member while the outside of the pipe is externally threaded and is the male member of the joint.
It is desirable in connecting together lengths of oil well pipe that the coupling satisfy several functions. One of these functions is to provide leak resistance from internal or external pressure. It is also desirable that the coupling have the capability to resist tension and thereby prevent the connection from pulling apart. Additionally, the coupling preferably should have resistance to torsion in order to keep the pieces of pipe from rotating further into or out of the coupling. Additionally, a coupling preferably should have structural rigidity to avoid yielding under bending tension or compression stresses or any combination thereof.
In the past, a number of different devices have addressed the problem of leak resistance in threaded and coupled oil well connections. The present invention is a coupling that provides leak resistance in a manner different than any of the prior art.
Generally a threaded and coupled connection includes a pin thread which is cut on the outside diameter of each end of a length of oil well pipe. The mating coupling is threaded on the inside diameter for both ends of the coupling. In the prior art the inside threads of the coupling meet or overlap in the center of the coupling. The threads in the center of the coupling are deemed imperfect and thus are not intended to engage the mating pin threads on the pipe section. Generally, a gap of 1/2 inch to 1 inch remains between the two sections of pipe after makeup within the coupling.
In the oil industry, it is common to screw a coupling onto one end of each length of pipe at the point of manufacture. The present invention seeks to analyze the coupling from the point of view of its initial installation onto the piece of pipe at the threading plant. At the threading plant, the coupling is gripped on its outside surface and then screwed onto the mating pin end of the pipe section. This gripping action on the outside of the coupling results in compressive and torsional stresses in the coupling. The result is that a portion of these stresses is transferred to the pin member and may cause yielding due to compressive and torsional loading during makeup. A serious problem with these stresses is damage in the form of thread galling or permanent deformation of the threads.
When the first end of a coupling is made up with a section of pipe, hoop stresses are created in the coupling that are distributed throughout the full length of the coupling. The stress pattern that is created is positive on the made up side of the coupling and becomes negative or compressive on the open end of the coupling. When the end of the coupling is then made up with the second pin member, these negative or compressive stresses in the coupling become positive and tend to reduce the stresses on the first end of the coupling. In other words, during makeup of the second end of the coupling, the stresses generated at the second end during makeup tend to negatively impact the stresses which resulted from makeup of the first side of the coupling. As a result, the stresses in the coupling after final makeup of both ends are not uniformly distributed. Each side of the coupling will have different stress levels. Additionally, the stress at the center of the coupling will not be eliminated upon final makeup of the coupling.
The stress patterns that result in the coupling cause different leak resistance on either side of the coupling. The differential stresses would be minimized if both sides of the coupling were made up simultaneously or all of the machining was perfect on the coupling and pipe threads. However, neither simultaneous makeup nor perfect machining is possible or practical. The imperfect thread area in the center of a coupling is subject to additional high hoop stresses and expansion as a result of internal pressure. Resistance to expansion at that area of the coupling, therefore, is critical. The area of the coupling threads that is critical to preventing leakage is the portion directly adjacent to the lead threads of the ends of each pipe section.